The invention relates to ballast circuits for operating high-intensity-discharge lamps and in particular to a novel ballast circuit to regulate lamp power over a wide range of supply and lamp voltages.
High-intensity-discharge lamps consist of tubes in which electric arcs in a variety of materials are produced. An outer glass envelope provides thermal insulation in order to maintain the arc tube temperature. The temperature of the arc tube influences the color of the light produced and the life expectancy of the lamp. A ballast circuit is used to provide a high voltage to initiate an arc in the arc tube and supply power to maintain the arc. By regulating the power supplied to the lamp, the arc tube temperature can be controlled. Examples of high-intensity-discharge lamps include metal halide and high-pressure sodium-vapor lamps. Recent advances in high-intensity-discharge lamps have improved the color, start up time, and life expectancy opening doors to new markets previously dominated by incandescent lamps. One draw back of the new high-intensity-discharge lamps is that the new lamps require tighter power supply regulation.
A typical high-intensity-discharge ballast circuit is shown in FIG. 1. The circuit consists of an inductor 250 in series with the lamp 256 and a capacitor 254 shunting the voltage supply 252 for power factor correcting. The inductor is typically sized to provide optimal power to a nominal lamp at a given supply voltage. The power supplied to the lamp (Plamp) will be the voltage across the lamp (Vlamp) multiplied by the current through the lamp (Ilamp). Appling Ohms law, Ilamp equals Vlamp divided by the lamp resistance (Rlamp). Summing the voltages around the circuit, supply voltage (Vsupply) will equal the voltage across the inductor (Vinductor) plus Vlamp. Rewriting the power equation yields Plamp=(Vsupplyxe2x88x92Vinductor)2/Rlamp. As the lamp ages, it""s resistance may decrease. Many utility companies consider variations in supply voltage up to ten percent from nominal typical and acceptable. Changing loads on the power supply may cause the voltage to vary more than the typical ten percent in some applications. As shown in the above lamp power equation, variations in lamp resistance and supply voltage can cause the power supplied to the lamp to vary. In many lighting applications, it is desirable to have a lamp emitting a constant light intensity, which requires providing constant power to the lamp. In addition to providing consistent light intensity, a constant power supply may increase the life of the lamp. In other applications it may be desired to operate the lamp at various constant power levels to dim, reduce power consumption, or change the color of the lamp.
Electronic ballasts are available today, which provide constant power and dimming capabilities. However, these ballasts are much more expensive. The increased expense may be due to the additional circuitry required to sense, calculate, and regulate the power supplied to the lamp. Of the three circuits, the one used to calculate the power in the lamp is usually most expensive. As noted above, the power supplied to a lamp may be calculated by multiplying the voltage across the lamp times the current passing through the lamp. Circuits to multiply generally are complicated and require a high level of precision accounting for the high cost.
What is therefore needed is a ballast circuit that will provide constant power utilizing an inexpensive power regulating circuit, and provide for optional dimming of the lamp.
One aspect of the invention provides a method of controlling power to a high-intensity-discharge lamp. Voltage across and current through the lamp are determined. Power to the lamp may be approximated using the voltage and current. Power to the lamp can be regulated based on a comparison of the approximated power and a predetermined value.
Current through the lamp is determined by converting the current to a representative voltage. The voltage across the lamp is determined by scaling the lamp voltage. Lamp power is approximated by the summation of the representative voltage and the scaled voltage. A comparison is made whether the approximated power is greater or less than the predetermined value.
Another aspect of the invention provides a system of controlling power to a high-intensity-discharge lamp. Voltage across the lamp is determined by a voltage sensor. Current through the lamp is determined by a current sensor. A control circuit is operatively connected to the current sensor and voltage sensor. The control circuit approximates a lamp power based on input from the sensors. The control circuit compares the lamp power against a desired level and regulates lamp power based on the comparison. The current sensor comprises a resistor connected in series with the lamp. A signal conditioning circuit scales and filters the output of the current sensor. The voltage sensor comprises a voltage divider network shunting the lamp. The voltage divider includes a voltage-limiting network. The control circuit includes a summing circuit. The summation circuit includes a filter and a plurality of rectifiers. The control circuit includes a voltage reference signal generator. The signal generator produces a saw tooth waveform synchronized with the sensed current and twice the frequency of the sensed current. The control circuit includes a current limiting component. The control circuit includes a comparator circuit.
The foregoing and other features and advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawing.